Known sensor units operate according to the pulse echo principle, in which the duration of a pulse in a pulse train determines the periods of time in which ultrasound waves or microwaves are emitted into the detection area of the sensor. An object reflects the waves as an echo to the sensor. The signals received by the sensor are analyzed to determine whether an object that exceeds a certain size is located in a certain detection area. Measuring cells, for example, which subdivide a close range, for example, a range from 0 meters to 7 meters using a grid spacing of 3 cm, form detection areas.
In pulse echo sensors, the relationship between energy E reflected by the obstacle and distance d of the sensor unit to the object reflecting the waves is not linear. The following approximate relationship applies:E=1/dx  (1)where x is an exponent dependent on the design of the sensor unit.
FIG. 1 shows a schematic diagram of a known circuit 10 for specifying the values of a gain factor VF1 of an amplifier 12. Amplifier 12 is a controllable analog amplifier, e.g., of the type CA3080. A sensor signal on an input line 14 is amplified as a function of gain factor VF1. An amplified sensor signal is output to an output line 16, to which the output of amplifier 12 is connected. Output line 16 is connected to an analyzer circuit (not shown).
Gain factor VF1 is determined by the current at a control input of the amplifier. The control input is connected to a control line 18, which is connected to a tap 20 of a voltage divider, having resistors R1 and R2, located between ground and positive operating voltage VCC. A capacitor C is connected in parallel with resistor R2. Resistors R1, R2 and capacitor C form an RC element 22.
In the known sensor unit, 256 measuring cells spaced at 3 cm, for example, are measured during a detection cycle. For example, 300 wave packets are emitted within a measuring cell. The signals generated by the reflected waves are added up in the sensor unit. The processing of a measuring cell is completed after approximately 50 μs. The time of one detection cycle for all measuring cells is approximately 10 ms.
In the known sensor, the sequence of the detection areas and their duration is fixedly predefined by the design of a sensor unit. The values of gain factor VF1 are adjusted to the measuring ranges and thus also fixedly predefined. The gain factor of amplifier 12 is set by RC element 22. The application of a voltage to RC element 22 is synchronized with the beginning of a detection cycle. During the detection cycle, the voltage across RC element 22 increases exponentially as the distance of the detection area to the sensor unit increases. Due to this measure, the same object generates approximately the same output voltage across the amplifier in all detection areas. This simplifies further processing of the sensor signal.